platform/surface: Split memcpy() of struct ssam_event flexible array

To work around a misbehavior of the compiler's ability to see into
composite flexible array structs (as detailed in the coming memcpy()
hardening series[1]), split the memcpy() of the header and the payload
so no false positive run-time overflow warning will be generated.

[1] https://lore.kernel.org/linux-hardening/[e-mail]

Cc: Maximilian Luz <[e-mail]>
Cc: Hans de Goede <[e-mail]>
Cc: Mark Gross <[e-mail]>
Cc: [e-mail]
Signed-off-by: Kees Cook <[e-mail]>
Reviewed-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Maximilian Luz <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Hans de Goede <[e-mail]>

drivers/platform/surface/surface_acpi_notify.c

···
 	INIT_DELAYED_WORK(&work->work, san_evt_bat_workfn);
 	work->dev = d->dev;
 
-	memcpy(&work->event, event, sizeof(struct ssam_event) + event->length);
+	work->event = *event;
+	memcpy(work->event.data, event->data, event->length);
 
 	queue_delayed_work(san_wq, &work->work, delay);
 	return SSAM_NOTIF_HANDLED;